How do I calculate the phase difference for this?

[Daniel Pinto]

Homework Statement

Light with wavelength 100nm is incident perpendicularly from air on a film 0.500μm thick
and with refractive index 2.5. Part of the light is reflected from the first surface of the film,
and part enters the film and is reflected back at the second surface, where the film is again
in contact with air. What is the phase difference?

Homework Equations

phase difference = (2pi*deltaL)/lambda

The Attempt at a Solution

found the angle then added the vectors, used cosine rule and got an answer similar to it.

 

[ehild]

Welcome to PF!

Homework Statement

Light with wavelength 100nm is incident perpendicularly from air on a film 0.500μm thick
and with refractive index 2.5. Part of the light is reflected from the first surface of the film,
and part enters the film and is reflected back at the second surface, where the film is again
in contact with air. What is the phase difference?

Phase difference between what two things?

Homework Equations

phase difference = (2pi*deltaL)/lambda

This is the phase change of a wave between two wavefronts delta L distance apart.

The Attempt at a Solution

found the angle then added the vectors, used cosine rule and got an answer similar to it.

What angle did you find and what vectors did you add an why? And the answer was similar, to what? Show your work and result, please.